In subscription television systems such as, for example, the system disclosed in U.S. Pat. No. 3,824,332 (Horowitz), the picture information is inverted in a random manner on a field basis while maintaining the synchronizing signals continuously at the same polarity. The present invention is, of course, not limited to such systems, but may be applied wherever a reversal of polarity of the television signal occurs. It will, however, be described as employed in the system disclosed in the above mentioned patent.
In such a subscription television system, the picture information is, of course, reconstituted at the decoders by reversing the inversion which has taken place at the encoder. In systems of this type, the following difficulties have been encountered.
1. The electronic reversing switch producing the inversion introduces spikes or transients in the signal. Such transients provide decoding information to would be pirates.
2. The reconstitution process in the decoder as mentioned above requires a switching operation to be performed between two video signals of opposite phase. The introduction of transients in the switching operation can adversely affect the operation of the home television receivers.
3. Mismatches in the DC components of the two signals, namely the reversed and the synchronizing signals, result in annoying flicker on the home television screen.
The above mentioned difficulties will be described in greater detail with reference to FIG. 1.
Consider one horizontal line of a conventional video signal as shown in FIG. 1(a) and the same signal with the picture information inverted as in FIG. 1(c). For convenience, these are shown as monochrome signals. The inverting pulse which produces the inversion required for scrambling is shown in FIG. 1(b) and the signal with inverted picture information in FIG. 1(c). If the pulse in FIG. 1(b) does not start and stop exactly coincident with blanking, the resulting signals are shown at FIG. 1(d) and FIG. 1(e). In one case, part of blanking and in the other, part of information is present in the wrong place. Information is therefore available to bypass the entire coding system. Also, because of the abrupt transition required by the inversion in FIG. 1(c), it is difficult to avoid an overshoot in the switching circuitry.
To reconstitute the signal in its proper format, a switching operation is again carried out between the sync portion and the video portion of FIG. 1(c). If the instant of switching is exactly coincident with that of the original coded signal (and at exactly the same rate of change), the signal in FIG. 1(a) will be recovered. However, if the switching occurs either too early (A) or too late (B), huge spikes will result, as shown in FIGS. 1(g) and 1(h). It is, of course, impossible to achieve perfect timing and consequently spikes in the decoded signal are inherent in the system.
The reconstituted signal at the decoder, that is the signal whose picture signal is again noninverted relative to the synchronizing signal, should have exactly the same DC level as the original, otherwise an annoying flicker results. Without the system of the present invention, it is very difficult to maintain this DC level over extended periods of time.